1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine or a laser beam printer, using an electrophotographic system or an electrostatic recording system that develops an electrostatic latent image formed on an image bearing member by using a developer, which includes toner and a carrier.
2. Description of the Related Art
Conventionally, an image forming apparatus, such as a facsimile, a printer, or a copying machine using an electrophotographic system has generally formed images by repeatedly performing electrophotographic processes (a charging process, an exposure process, a developing process, a transfer process, and a cleaning process) on an electrophotographic photoreceptor (photoreceptor) that is an image bearing member. The image forming apparatus includes a charging device, an exposure device, a developing device, a transfer device, a cleaning device, a fixing device, and the like in addition to a photoreceptor. The charging device (charging process) uniformly charges the photoreceptor with a predetermined polarity and an electric potential. The exposure device (exposure process) forms an electrostatic latent image on the charged photoreceptor. The developing device (developing process) visualizes the electrostatic latent image, which is formed on the photoreceptor, as a toner image by using a developer. The transfer device (transfer process) transfers the toner image to a recording material such a paper sheet from the surface of the photoreceptor. The cleaning device (cleaning process) cleans the surface of the photoreceptor by any removing residues (residual developer and transfer residual toner) remaining on the photoreceptor after the transfer process. The fixing device (fixing process) fixes the toner image onto the recording material.
Further, in recent years, a tandem image forming system has been used due to the demand for the increase in speed of the full-color image forming apparatus. The tandem image forming system includes photoreceptors, charging devices, exposure devices, and developing devices, which correspond to four colors (for example, yellow, magenta, cyan, and black), and these devices are disposed in parallel so that an image is formed in each of units. Since images corresponding to four colors can be simultaneously formed by the tandem image forming system, it may be possible to output an image at high speed.
Meanwhile, it is known that corona products, which are generated due to the existence of high-voltage members, such as charging members and transfer members, in the apparatus, are attached to the surface of the photoreceptor as foreign materials. Accordingly, particularly, resistance is lowered under a high-humidity environment, so that the formation of a clear electrostatic latent image is hindered and the deterioration of image quality is caused (image deletion). As factors that cause the image deletion, there are ingredients that form nitrate ions by the oxidation of nitrogen contained in the air as well as the generation of various metal oxides or oxygen compounds during corona discharge. Further, since the corona products are attached to the surface of the photoreceptor, a thin film (hereinafter, referred to as a filming layer) is formed on the surface of the photoreceptor. Accordingly, the photoreceptor absorbs moisture under a high-humidity environment, so that resistance is lowered. For this reason, the formation of a clear electrostatic latent image is hindered. These become factors that cause the deterioration of image quality.
A drum heater is mounted in the photoreceptor in order to suppress the image deletion (Japanese Patent Application Laid-Open (JP-A) No. 9-22168).
However, the image forming apparatus has been reduced in size, manufacturing cost, and energy consumption in recent years. Accordingly, the mounting of the drum heater causes problems, such as the securing of the space required for the disposition of the heater, the increase of manufacturing cost, and the increase of power consumption.
Further, there has been a method of suppressing image deletion by scraping attached corona products together with the surface layer of a photoreceptor while rubbing the surface of the photoreceptor by a cleaning device. However, as disclosed in JP-A No. 2006-53168, the hardness of the surface of a photoreceptor has been increased in recent years in order to increase the life of a photoreceptor. For this reason, it has been difficult to remove corona products by rubbing of the surface of the photoreceptor.
Furthermore, as disclosed in JP-A No. 2000-47545, there has been known a method of removing discharge products by depositing abrasive particles on a cleaning device, which comes into contact with a photoreceptor, from a developing device via the photoreceptor, after abrasive particles for abrading the surface of the photoreceptor are mixed in a developer in the developing device. The abrasive particle has an opposite-polarity that is opposite to a polarity of toner (for example, if toner has a negative polarity, the abrasive particle has a positive polarity). Accordingly, the abrasive particles are developed in a blank (fog removing bias, Vback) on the sheet, are hardly transferred since having the opposite-polarity that is opposite to the polarity of toner, and are collected by the cleaning device.
However, as described in JP-A No. 2000-47545, image deletion conventionally has been suppressed by unilaterally supplying abrasive particles to the cleaning device and collecting the abrasive particles on the cleaning member of the cleaning device that comes into contact with the photoreceptor. However, there were problems in the above-mentioned tandem full-color image forming system. That is, abrasive particles, which are generated from an image forming unit (first image forming portion) provided on the upstream side in a moving direction of a member to be transferred, are retransferred to an image forming unit (second image forming portion), which is provided on the downstream side, through the member to be transferred, so that the retransferred abrasive particles are attached to the cleaning member. For this reason, many abrasive particles are attached to the cleaning member of the image forming unit that is provided on the downstream side. As a result, all abrasive particles are not collected on the cleaning member of the image forming unit that is provided on the downstream side, so that abrasive particles contaminate a charging member and the photoreceptor. Accordingly, image defects have been caused or the life of the photoreceptor has been shortened.
Furthermore, the tandem image forming system generates not only transfer residual toner that is generated in each of the image forming units but also toner (retransfer toner) that is generated by the partial retransfer of the toner image formed in the image forming unit (first image forming portion) provided on the upstream side. If the retransfer toner is generated, a large amount of toner is attached to the cleaning member of the image forming unit (second image forming portion) that is provided on the downstream side. As a result, the amount of abrasive particles attached to the cleaning member of the image forming unit provided on the downstream side is decreased, therefore, image deletion has been caused.